Various industrial processes often require that a mass of porous material to washed in order to remove chemicals or other impurities. For example, this need appears in the sugar industry, where sugar is washed from bagasse; in the textile industry, where excess dyes are washed from the fabric; in mining, where impurities are washed from ore; and in the paper industry, as follows.
In a standard paper production line, wood chips are cooked with chemicals in aqueous solution, the precise composition of the cooking chemicals depending on the particular process. This step, normally carried out in a digester under heat and pressure, breaks down the wood by dissolving the organic compounds that hold the cellulose fibers together.
The mixture of pulp, spent cooking chemicals, and organic materials, collectively known as stock, is then fed to a series of washers. The most common type of washer system includes a rotary vacuum drum onto which the stock is spread. The drum is perforated, and a vacuum maintained inside causes the separation of liquid from the pulp. The mixture assumes the form of a pulp mat which is still impregnated with chemicals and organics. A washer usually disposed above, and extending axially along the drum directs water at and through the pulp mat to remove these substances. A typical installation would use three washer drums in sequence, with wash water being flowed counter-current to the direction of the pulp movement so that the final washing stage uses clean water. An additional washing stage to remove bleaching chemicals is required if the washed pulp is subsequently bleached.
The effluent from the washers, comprising water, spent cooking chemicals, and organic materials is referred to as liquor. In a kraft (or sulfate) process, it is called black liquor; in a sulfite process, red liquor. Typical liquor contains approximately 15% solid material. It is desirable to separate this solid material from the water to allow reuse of the inorganic pulping chemicals, and to eliminate the environmental problem of disposing of the liquor.
Evaporation is the standard separation method, with the liquor being passed through a series of evaporators, in which steam is passed countercurrent to the liquor flow. In this way, the liquor is concentrated until it contains approximately 60% solids, at which point it is burnt in a boiler. The organic materials provide the fuel value to generate the steam, and the inorganic chemicals smelt out the bottom of the boiler. In atypical paper mill, the steam from the liquor recovery part of the cycle supplies most of the mill's steam needs.
It is apparent that the more dilute the liquor, the more energy must be expended in evaporating the water in order to recover the solids. This is energy that is therefore unavailable for other energy needs of the paper mill. At the same time, it is necessary to efficiently remove the chemicals from the pulp to provide a satisfactorily clean pulp. A thorough washing militates toward the formation of dilute liquors.
Two standard types of paper pulp mat washers are the weir and the whistle shower. In the former, the water stored in a reservoir above the rotary vacuum drum on which the pulp mat is disposed is allowed to overflow a weir that extends axially along the entire length of the drum. Thus, in principle, a sheet of water falls along the entire width of the pulp mat. A difficulty with this type of a shower, is that the weir shower, a relatively long piece of equipment supported at its ends, has a tendency to sag, and is further prone to misalignment. In such cases, more water overflows the weir at its lowest portions than elsewhere, thereby washing some portions of the pulp mat less effectively than others. In a whistle shower, nozzles disposed above and axially along the pulp mat direct water at the mat. A difficulty with this type of washer is that the nozzles can easily become plugged, resulting in incomplete washing of the pulp mat. Satisfactory washing with weir or whistle showers typically requires three showers arranged in a parallel configuration above the drum.
Thus, there is a need for a washer having a high efficiency and not subject to problems that result in only a portion of the pulp mat's being washed.